10 
BULLETIN 551. U. S. DEPAETMEXT OF AGRICULTURE. 
phases of soil composition concerning which it is extremely desirable 
to reach conclusions, but with this limitation in mind it is thought 
that the following topics may be discussed from the point of view of 
chemical composition: Relation of soil to subsoil; extreme variation 
in composition: variation in soils of the same province: variation in 
soils of the same type ; and the relation of the limit of error in analysis 
to quantities of constituents. 
RELATION OF SOIL TO SUBSOIL. 
It has been commonly observed that where there is a distinct sub- 
soil, differing in color and texture from the surface soil, there may 
be a marked difference in the chemical composition of the two and 
that this difference is most marked in the content of silica, iron, and 
alumina. Usually it is found that the silica is higher in the surface 
soil and the iron and alumina in the subsoil. 
The analyses presented here are in accord with this experience. 
Thirty-four locations are represented by soil and subsoil, and in all 
but two cases the silica is higher in the surface soil. The iron is 
higher in the subsoil in all but five cases, and the alumina in all but 
one. 
Allowing a limit of error for all determinations of 0.05 per cent 
and regarding all figures that do not differ by more than 0.05 as equal, 
the relations of soil to subsoil in composition are summarized in 
Table 4. 
In this table are stated the number of locations where soil and 
subsoil are the same in composition with regard to any constituent 
and the number of locations where they were found to differ by 
amounts of 0.05 to 0.5, 0.5 to 1, and more than 1 per cent. 
Table 4. — Comparison of soils and 
as regards chemical composition. 
Group. 
Si0 2 . 
Fe 2 3 . 
A1 2 3 . 
CaO. 
MgO. 
K 2 0. 
Xa 2 0. 
P;0 5 . 
S0 3 . 
Soil higher, more than 1 per ' 
3 
1 
1 
Soil hi£rhf>r, 0.5 to 1 per cent.. 
Soil higher, 0.05 to C5 per 
cent 
Soil and subsoil equal 
4 
1 
19 
6 
1 
3 
7 
21 
3 
5 
12 
IS 
22 
5 
3 
2'J 
2 
5 
24 
Subsoil higher, 0.05 to 0.5 
5 
Subsoil higher, 0.5 to 1 per 
2. 
11 
l c 
1 
32 
1 
Subsoil higher, more than 1 
per cent 
r 
EXTREME VARIATION. 
The soils analyzed represent a wide variation in kind of material 
from which they have been derived, processes of formation, topog- 
raphy, and present climatic conditions, and probably represent for 
some of the constituents nearly the extremes in composition of soils 
in this countrv. 
